fix _require_scratch test for extN, resierfs, gfs2, and btrfs

fix _require_scratch test for extN, resierfs, gfs2, and btrfs

The extN, reiserfs, gfs2, and btrfs filesysytem types should use the
same check for a block device as XFS and UDF, and not the test for
NFS, which was checking for host:/foo/bar/baz when checking for a
scratch device.

Also, the NFS logic was also incorrect, in that it would allow a
zero-length SCRATCH_DEV to continue.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>